This invention relates generally to an electrostatographic printing machine, and more particularly concerns electrostatographic reproduction machines including dual mode development unit control apparatus and method for optimizing machine operating conflicts under relatively high and sustained area toner coverage, or toner area coverage conditions.
In an electrostatographic printing machine, a photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon in irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced.
After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing developer material containing charged toner particles, for example, black toner particles, into contact therewith. Developer material can be single component comprising only of charged toner particles, or it may be dual component comprising carrier particles and toner particles that are triboelectrically charged when admixed or mixed with the carrier particles. In either case, bringing the developer material into contact with the latent image forms a toner image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is then separated from the photoconductive member and the toner powder is fed on the copy sheet through a fusing apparatus where it is heated to permanently affix it to the copy sheet, thus forming a black and white copy of the original document.
Multi-color electrostatographic printing machines which use multi-colored toners are substantially identical in each color image forming process to the foregoing process of black and white printing which uses only black toner. However, rather than forming a single latent image on the photoconductive surface, several single color latent images corresponding to color separated light images of the original document are recorded thereon. Each single color electrostatic latent image is developed with toner particles of a color complementary thereto. This process may be performed in a single pass, or in multipasses during which image formation is repeated a plurality of cycles for differently colored images using their respective complementarily colored toner particles to form color toner images. Each single color toner powder image is transferred to a copy sheet in superimposed registration with the other toner powder images.
This creates a composite multi-layered toner powder image on the copy sheet. The copy sheet is separated from the photoconductive member and, thereafter, the multi-layered toner powder image on the sheet is fed through a fusing apparatus and permanently affixed to the copy sheet, thus creating a color copy of the original multi-color document. In a black and white or multi-color electrostatographic printing machine, the copy sheet is typically brought into moving contact with the photoconductive member during toner powder image transfer to the copy sheet. A sheet transport apparatus is typically provided for receiving the copy sheet incrementally as it is incrementally separated from the photoconductive member, and for transporting the copy sheet towards and into the fusing apparatus.
Multipass color electrostatographic reproduction machines are usually designed so that they each have a controller that includes a fixed development unit operating or on time control algorithm or method which is optimized for typical operating conditions of each machine. For example, in continuous operation of a four color four pass electrostatographic reproduction machine, each development unit typically adds, admixes or mixes and develops toner for approximately 25% of the total machine operating time. Unfortunately, such a controller usually requires performance tradeoffs when the machine is operating and producing reproductions each having high area toner coverage, and consequently requiring high rates of toner depletion and toner replenishment (in other words high rates of toner throughput) during the fixed operating times. In order to maintain the concentration of toner particles within the developer material at a desired level that would insure the quality of subsequent images, the depleted toner particles must be replenished with fresh toner particles. Such fresh toner particles must then be admixed with the carrier particles in order to properly charge them triboeletrically.
Tradeoffs are necessary because it is very difficult to design toner materials and compact mixing apparatus which enables high image quality while replenishing toner at 100% of the maximum depletion rate (that is, at 100% area coverage). Maintaining constant toner concentration at area toner coverage levels above 50%, the resultant high replenishment and admix rates often result in undesirable side effects such as background development and excessive contamination. One possible way to avoid these side effects is to limit the maximum toner dispense rate to approximately 50% of the maximum toner depletion rate. In sustained operation at high area toner coverage levels, this will cause the toner concentration to decline by a large amount and will produce objectionable degradation of image quality. Another common solution is to suspend imaging when the toner concentration has fallen by a small amount and allow the development unit and toner dispenser to run and mix until toner is replenished to normal levels, thereby sacrificing productivity (measured by prints per unit time). This however may result in customer dissatisfaction with the operation of the machine, particularly with small development unit capacity machines.
Operating a four color multicolor printing machine with toner depletion and replenishment as above, involves a number of conflicts. On the one hand, in order to insure optimum toner concentration as well as optimally charged toner particles from toner replenishment and subsequent necessary admix or mixing, it would be desirable to keep all development units constantly replenishing and mixing toner particles 100% of the time when this option is enabled by the process architecture. This would spread necessary toner replenishment over the longest possible time and at the lowest possible replenishment rate. This desire is counteracted on the other hand by a need to minimize developer material aging from over mixing, and by a desire to switch development units off whenever possible in order to minimize their power consumption and audible noise.
Any level of fixed development unit replenishment and mixing "on" time in a multipass electrostatographic reproduction machine results in a compromise in performance at some toner throughput rate as determined by area toner coverage. If the development unit replenishment and mixing "on" time is minimized, then image quality or productivity must be compromised at high throughput and high area toner coverage levels. If the development unit replenishment and mixing "on" time is maximized, then developer life, noise, and power consumption must be compromised at low throughput.